Those skilled in the art of electrochemical devices, including without limitation, solid oxide fuel cells, oxygen separators, and hydrogen separators, recognize a need for improved seals at the interface between ceramic and metal parts utilized in these devices. For example, among solid oxide fuel cell designs, the planar stack (pSOFC) has received growing attention because its compact nature affords high volumetric power density—a design feature of particular importance in transportation applications. With the advent of anode-supported cells that employ thin YSZ electrolytes, these devices can be operated at reduced temperature (700-800° C.) and still achieve the same current densities exhibited by their high-temperature, thick electrolyte-supported counterparts, as described in B. C. H. Steele, A. Heinzel (2001) Materials for fuel-cell technologies, Nature, 414(X) 345-52. The entire contents of this, and each and every other patent, paper or other publication referenced herein is hereby incorporated into this disclosure in its entirety by this reference. The lower operating temperature not only makes it possible to consider inexpensive, commercially available high temperature alloys for use in the stack and balance of plant, but also expands the range of materials that can be considered for device sealing.
Because SOFCs function under an oxygen ion gradient that develops across the electrolyte, hermiticity across this membrane is paramount. In a planar design, this means that the YSZ layer must be dense, must not contain interconnected porosity, and must be connected to the rest of the device with a high temperature, gas-tight seal of the type shown in FIG. 1. One of the fundamental challenges in fabricating pSOFCs is how to effectively seal the thin electrochemically active YSZ membrane against the metallic body of the device creating a hermetic, rugged and stable stack. Typical conditions under which these devices are expected to operate and to which the accompanying YSZ-to-metal seal will be exposed include: (1) an average operating temperature of 750° C.; (2) continuous exposure to an oxidizing atmosphere on the cathode side and a wet reducing gas on the anode side; and (3) an anticipated device lifetime of 10,000+ hours.
Two techniques are typically used by those skilled in the art to seal a planar stack; glass joining and compressive sealing. Inherent advantages and limitations are found with each method. For example, glass joining is a cost-effective and relatively simple method of bonding ceramic to metal. However, the final seal is typically brittle and non-yielding, making it particularly susceptible to fracture when exposed to tensile stresses such as those encountered during non-equilibrium thermal events or due to thermal expansion mismatches between the glass and joining substrates as described in K. Eichler, G. Solow, P. Otschik, W. Schaffrath (1999) BAS (BaO.Al2O3.SiO2) glasses for high temperature applications, J. Eur. Cer. Soc., 19(6-7) 1101-4 and Z. G. Yang, K. S. Weil, D. M. Paxton, K. D. Meinhardt, J. W. Stevenson (2003) Considerations of glass sealing solid oxide fuel cell stacks, in: J. E. Indacochea, J. N. DuPont, T. J. Lienert, W. Tillmann, N. Sobczak, W. F. Gale, M. Singh (Eds.) Joining of Advanced and Specialty Materials V, ASM International, Materials Park, Ohio, 40-48.
In addition, as the initial glass seal begins to devitrify during the first few hours of high-temperature exposure, its engineered thermal expansion properties change significantly, ultimately limiting the number of thermal cycles and the rate of cycling that the stack is capable of surviving. Over time additional problems arise as the sealing material, typically barium aluminosilicate-based, reacts with the chromium- or aluminum oxide scale on the faying surface of the interconnect and forms a mechanically weak barium chromate or celsian phase along this interface as described in Z. G. Yang, K. S. Weil, K. D. Meinhardt, J. W. Stevenson, D. M. Paxton, G.-G. Xia, D.-S. Kim (2002) Chemical compatibility of barium-calcium-aluminosilicate base sealing glasses with heat resistant alloys, in: J. E. Indacochea, J. N. DuPont, T. J. Lienert, W. Tillmann, N. Sobczak, W. F. Gale, M. Singh (Eds.) Joining of Advanced and Specialty Materials V, ASM International, Materials Park, Ohio, 116-24.
In compressive sealing, a compliant, high-temperature material is captured between the two sealing surfaces and compressed, using a load frame external to the stack. Because the sealing material conforms to the adjacent surfaces and is under constant compression during use, it forms a dynamic seal. That is, the sealing surfaces can slide past one another without disrupting the hermeticity of the seal and coefficient of thermal expansion (CTE) matching is not required between the ceramic cell and metallic separator. Unfortunately, this technology remains incomplete due to the lack of a reliable high-temperature sealing material that would form the basis of the compressive seal. A number of materials have been considered, including mica, nickel, and copper, but each has been found deficient for any number of reasons, ranging from oxidation resistance in the case of the metals to poor hermeticity and through-seal leakage with respect to the mica as described in S. P. Simner, J. W. Stevenson (2001) Compressive mica seals for SOFC applications, J. Power Sources, 102 (1-2) 310-6.
An additional difficulty is in designing the load frame, as it must be capable of delivering moderate-to-high loads in a high-temperature, oxidizing environment over the entire period of stack operation. Material oxidation and load relaxation due to creep, as well as added expense and additional thermal mass are all issues of concern with this seal design.
The inventors of the present disclosure recently developed an alternative method of ceramic-to-metal brazing specifically for fabricating high temperature solid-state devices such as oxygen generators described in J. S. Hardy, J. Y. Kim, K. S. Weil (in press) Joining mixed conducting oxides using an air-fired electrically conductive braze, J. Electrochem. Soc. Vol. 151, No. 8, pp. j43-j49 and U.S. patent application Ser. No. 10/334,346. Referred to as air brazing, the technique differs from traditional active metal brazing in two important ways: (1) it utilizes a liquid-phase oxide-noble metal melt as the basis for joining and therefore exhibits high-temperature oxidation resistance and (2) the process is conducted directly in air without the use of fluxes and/or inert cover gases. In fact, the strength of the bond formed during air brazing relies on the formation of a thin, adherent oxide scale on the metal substrate. The technique employs a molten oxide that is at least partially soluble in a noble metal solvent to pre-wet the oxide faying surfaces, forming a new surface that the remaining molten filler material easily wets. A number of metal oxide-noble metal systems are suitable, including Ag—CuO, Ag—V2O5, and Pt—Nb2O5 as described in Z. B. Shao, K. R. Liu, L. Q. Liu, H. K. Liu, S. Dou (1993) Equilibrium phase diagrams in the systems PbO—Ag and CuO—Ag, J. Am. Cer. Soc., 76 (10) 2663-4, A. M. Meier, P. R. Chidambaram, G. R. Edwards (1995) A comparison of the wettability of copper-copper oxide and silver-copper oxide on polycrystalline alumina, J. Mater. Sci., 30 (19) 4781-6, and R. S. Roth, J. R. Dennis, H. F. McMurdie, eds. (1987) Phase Diagrams for Ceramists, Volume VI, The American Ceramic Society, Westerville, Ohio.
While advances in sealing techniques such as the brazing technique described above have improved the performance of ceramic to metal joints in high temperature environments typical of electrochemical devices such as solid oxide fuel cells, there still exists a need for further improvements in these joint that allow them to operate over multiple cycles while maintaining a hermetic seal between the metal and ceramic parts.